Serum alpha 1-antitrypsin Pi types and trypsin inhibitory capacity (TIC) were measured in 72 patients with lung cancer and in 196 patients with abnormal sputum cytology but no clinical evidence of lung cancer to determine if a genetic deficiency of alpha 1-antitrypsin (AAT) is a predisposition to lung cancer. The distribution of Pi types in these two groups of patients and healthy adults is similar. Serum TIC and AAT concentrations are elevated in lung cancer patients. However, patients with abnormal sputum cytology and no clinical lung cancer have normal levels of serum TIC and AAT. A genetic deficiency of AAT probably does not produce a state of increased susceptibility to the carcinogenic effects of respiratory carcinogens such as tobacco smoke.